A.C. electrical motors are widely used in commercial, industrial, domestic and other applications where rotating power is needed. The simplest and generally most reliable motors for most uses have been the A.C induction motors, a widely used form being known as squirrel cage motors. However, there are a number of well-known short-comings met with in such induction motors. Most A.C. induction motors will draw excessively high starting currents, usually from 5 to 7 times, and sometimes up to 9 times, the amperage drawn when at full load and at full operating speed. Even a small induction motor of a horsepower or so driving a saw or large appliance will cause the electric lights to dim in the place where it is being run due to the large current being drawn during starting, and such heavy current drain can cause blown fuses and produce other adverse results. Therefore, for larger capacity A.C. induction motors, above about 10 horsepower, for example, special starters, which often cost as much as the motor itself, are often required to reduce such excessively high amperage starting currents to acceptable levels though this reduces the starting torque also. Induction motors also have poor power factor characteristics and this creates power line problems. Because of their poor power factor at low speed, they have difficulty starting loads with high inertia without severe motor overheating. Induction motors by their nature have zero torque at synchronous speed. To develop torque they must operate at less than synchronous speed, i.e. at slip speed. The amount of such slip, (the difference in speed between the rotating magnetic field of the stator and the mechanical speed of the rotor) depends on the motor design and the load, typically being 2-5% in a modern motor. This results in a shaft speed that varies with load as well as causing substantial electrical losses and undesirable heating of the motor's electrical windings and rotor cage. High resistance rotors can be used to reduce starting inrush current but at a proportionally higher operating slip.
As defined and well understood in the art, synchronous speed, Ns, in RPM for an A.C. motor is given by the formula: EQU Ns=120 f/p
where f is the A.C. frequency to the motor in cycles per second or Hertz (Hz), and p is the number of poles in the rotating magnetic field of the stator.
Conventional wound field synchronous motors operate at synchronous speeds, (zero slip), and can exhibit unity input power factor and higher efficiency than induction motors. They have the handicap of not being able to develop torque at any speed other than at synchronous speed and hence cannot start from standstill. In order to be able to start, modern synchronous motors usually have shorted windings in the rotor in an addition to the field windings on the poles. These shorted windings supply a starting torque much as in an induction motor to bring the machine to a speed close to synchronous. At this point the field windings are energized and the rotor of the synchronous motor is caused to pull into step with the rotating magnetic field of the stator. However, if the connected inertia of the load is higher than that which the motor can accelerate to synchronous speed in 1/2 cycle of input frequency, it will not synchronize and serious damage can result. This requirement severely limits the amount of inertial load that can be driven to synchronism by such a motor.
Permanent magnet synchronous motors have the same limitations as to inertial load, in addition to exhibiting a high pulsating torque as the motor approaches synchronous speed. Further, in order to allow space for an independent magnetic path for the squirrel cage windings required for starting and to avoid de-magnetization of the rotor permanent magnets during starting, the rotor structure of these permanent magnet synchronous motors is very complex and costly to manufacture when compared to a conventional squirrel cage induction motor or the motor of this invention.
Commercial permanent magnet synchronous motors are generally limited to sizes of 5 H.P. or less. Because of their high cost, in small sizes, commercial wound field synchronous motors are generally not manufactured in sizes less than about 50 to 100 H.P. It will be appreciated that a synchronous motor, either wound field or permanent magnet, is much more complex and several times more costly than an induction motor of similar capacity.
Under overload conditions a synchronous motor may drop out of synchronous speed and disastrous results may follow unless the input electrical power is immediately interrupted and/or the load dropped. When A.C. power from the power line is briefly interrupted, extremely high and damaging current surges can occur in a synchronous motor if attempts are made to restart it after the rotor field has drifted substantially out of phase with the A.C. line power.
In the art, small synchronous-induction motors known as hysteresis motors employing a solid rotor structure composed of a hardened steel or a similar easily magnetized permanent magnet material disposed over a non-magnetic core are built in small ratings. They can reach and operate at synchronous speeds, but due to their structure and reliance on hysteresis loss for their torque, they have such low torque that no substantial load for their size can be carried. Therefore their utility is primarily for such light loads as for operating clocks and small synchronous drive systems, usually of less than 1/10 Hp. Electrical handbooks list their power ratings as about 20 watts per pound of rotor.
There has been a long felt need for an A.C. electrical motor that can deliver continuous, substantial torque from standstill to a predetermined synchronous speed, being of simple low cost construction, having no brushes, commutators, or insulated rotor windings, that can start under substantial load without drawing more than roughly about 2 to 4 times the amperage drawn at full speed under full load, and without requiring costly starters so that they can be directly connected to a power line. Such motors should be able to easily reach synchronous speed regardless of the shaft and inertial loads reasonably applied to them within their rated capacity. A high electrical efficiency and substantially unity power factor under normal operating conditions would be desirable features for such electrical motors.